The speed of today's high-speed performance digital computers is dependent both on the speed of the IC chip and the compactness of the package on which the chip is mounted. The package connects the signals between the chips, provides power and cooling capability, as well as providing interconnects to other parts of the computer. By making the packaging circuits smaller, the signals have less distance to travel which translates into reduced processing time. As circuits get smaller, the way they are fabricated and the accuracy with which they have to be positioned becomes more challenging. Micro lithography for advanced ULSI fabrication is currently making a transition from using an i-line (365 nm) mercury lamp to a deep-UV excimer laser, primarily krypton fluoride (KrF), having a wavelength of 248 nm, as the illumination source. The KrF excimer laser is expected to become the primary exposure tool for printing sub-0.4 .mu.m design rule features in IC manufacturing, while Argon fluoride (ArF) with its 193 nm wavelength, is expected for use in sub-0.25 .mu.m design features.
In the micro lithography process, wafer steppers utilize precision positioning tables with interferometers and CCD optical recognition to provide accurate placement of design patterns in silicon wafers. The wafer is generally automatically loaded from a cassette into a wafer holder where it is automatically leveled, focused, and aligned to the mask. Each pattern on the part has to be overlaid with a defined accuracy on a previously-defined pattern. One pattern may be used as the mask for each part, wherein the excimer laser acts as a projection ablation tool to form the vias which interconnect the wiring levels of the circuit As the pattern changes, the mask is changed as the step-and-repeat process continues. These vias must be placed precisely on the features defined at the previous level, making the accuracy of the laser system critical. This accuracy can be affected by lens distortion, tool-to-tool errors, and particularly by alignment mark errors. Because of the increasingly smaller design rule features being required in the industry, changes in beam profile and direction cannot be tolerated as the result of misalignment or external forces acting to destabilize the alignment accuracy of the laser-stepper system.
Referring to FIG. 1, an example of a laser system support structure 10 of conventional design is shown, having a horizontal support frame 20 and rail cross members 30. The optical platform (not shown) and the laser system components mounted thereon are typically mounted on rail cross members 30 and supported equally by internal panel 35 and vertical frame members 40, which are tied into support frame 20 by brackets 45, shown in the closeup labeled A. Beam delivery interface mount 50, used to couple and hold the laser system to the wafer stepper's beam delivery equipment in a precise and stable manner, is generally mounted to frame 20 since, in this conventional design, this member is the most robust and will provide the most rigid support However, if a force F is applied to vertical frame member 40, a deflection in the frame occurs so that members AB and CD are biased in the direction of the acting force. As a result, interface mount 50 is likewise deflected due to its dependent mounting relationship with vertical frame members 40 via horizontal support frame 20 and brace 45. Because of the required alignment precision, even small vibrational forces acting on structure 10 in a manner as previously described, can adversely affect beam accuracy.